The invention relates to receiver protectors for radar systems in general, and more particularly to a receiver protector having high isolation and low insertion loss due to a unique microwave assembly configuration of at least one high power input protection stage and a sensitivity time controlled multi-level attenuator, both achieving rapid switching with a non-critical bias supply and attenuation levels which are invariant with temperature and insensitive to diode parameters.
A typical modern radar is depicted in the schematic block diagram of FIG. 1. Generally, a transmitter section 10 utilizes a power amplifier 12 to generate pulsed or continuous wave energy which is conducted through a conventional circulator 14 and radiated into space from an antenna system 16. Echo signals from targets within the range of the radar are received by the antenna system 16 conducted through the circulator 14 and coupled to a low noise amplifier (LNA) 18 where the received signals are conditioned and thereafter passed along to a receiver section 20. Additionally disposed in a radar between the circulator 14 and low noise amplifier 18 may be a section generally referred to as a receiver protector 22.
Normally, the power amplifier 12 generates signals for transmission with a very high power level. This transmission power is so great that any leakage through the circulator 14 to the LNA 18 may burn out or adversely affect its input stages. One of the functions of the receiver protector 22 is to adequately attenuate any transmission leakage signals to protect the sensitive input of the LNA 18 during the transmission times. In addition, the receiver protector 22 may also provide attenuation during the echo signal receiving time for maximum range performance.
In some radars, a sensitivity time control (STC) section is provided in the receiver protector stage 22 to attenuate the signal level of clutter and other signals at various pre-specified ranges of the radar. This sensitivity time control (STC) attenuation is usually switched between predetermined attenuation levels as a function of pre-specified radar range divisions. To simplify receiver processing, it is preferable that the various attenuation levels of the STC be maintained substantially constant with variations in temperature. In addition, the STC attenuations should be matched to the LNA 18 so that the noise figure and gain thereof remain substantially fixed between the various attenuation levels. It is also preferred that the RP/STC 22 be integrable with the LNA 18 in its implementation in order to minimize size, increase reliability and maximize performance.
An example of operation of the RP/STC 22 in connection with a pulse radar system is depicted in the time waveforms of FIG. 2. Referring to the waveform 2A, pulsed transmissions are shown at 30 and 32 with the interpulse period therebetween designated as 34. The radar echo pulses are received during the interpulse period 34 with the time of reception being indicative of target range. In time waveform 2B, it is shown that the receiver protector (RP) attenuation is provided at time increments 36 and 38 concurrent with the transmission pulses 30 and 32 with little or no attenuation being provided during the interpulse period 34.
The operation of a typical bilevel sensitivity time controlled attenuator is shown in the time waveform 2C. A conventional STC generally adds to the attenuation of the input protection state of the receiver protector during the transmission pulses (30 and 32) and continues to attenuate for a prespecified time during the interpulse periods thereafter. In the interpulse period 34, a first attenuation level of the STC, designated at 40 in waveform 2C, may last for a range division pre-specified from t0 to t1, for example. At range time t1, the STC may switch to a second attenuation level, designated at 42, and may maintain the new attenuation level through the range division t1 through t2, for example. During the interpulse period 34, say from t2 to the next transmission pulse, the STC may afford little to no attenuation of the received echo signals. As shown by the present example, the STC section exemplified may periodically repeat its bilevel attenuation operation for each transmission pulse as shown again in time waveform 2C starting at time t3.
A typical embodiment of an RP/STC is shown schematically in FIG. 3. That shown by the dashed lines 45 is representative of the input protection stage and that shown by the dashed line 47 is representative of the STC stage. A leakage power or received echo signal may enter the circuit at 46 and exit at 48. One input protection section of the stage 45 includes a PIN diode 50 coupled across the transmission line 58 and ground 60 with DC blocking capacitors 55 and 56 coupled in series with the input line 58 on either side of the shunted PIN diode 50. A bias line 62 may be coupled electrically to the anode of the PIN diode 50 and include an inductor 64 and capacitor 66 in series and shunt relationship, respectively, with respect to the bias line 62 and ground 60.
Downstream of the first protection section may be a second section of the stage 45 including another PIN diode 68 and an inductor 70 in parallel shunt connection with the transmission line 58 and ground 60. The STC circuit 47 may include a low level limiter diode 72 coupled across the main transmission line 58 and ground 60. An additional diode 74 with a 50.OMEGA. resistor 76 in series therewith may also be disposed in the circuit 47 coupled in parallel with the first limiter diode 72. The diodes 72 and 74 may be driven to conduction by a signal provided over another bias line 78 which includes an inductor 80 and capacitor 82 in a similar circuit configuration as that for the bias line 62. In this circuit, the attenuation of the circuit can be varied from near 0 to the order of 25 dB by varying diode current while the output impedance remains 50.OMEGA., providing a matched attenuator to the LNA 18. For a better understanding of an STC circuit, similar to the one described in connection with the embodiment schematically depicted in FIG. 3, reference is hereby made to an article entitled "A Matched Microwave Limiter" by G. Chao, found in the IEEE Transactions on Microwave Theory and Techniques, May, 1970.
In a typical operation of the RP/STC 22 of FIG. 3, the drive signals over corresponding lines 62 and 78 are provided during the time power is being transmitted like during the pulse widths 30 and 32 as shown in the time waveform 2A. Under these conditions, all of the diodes are conducting and providing attenuation of any power signals being leaked to the circuit 22 during transmission. After transmission, the drive signal 62 may be relieved, but the diodes 72 and 74 of the STC circuit may remain conducting to provide an attenuation state over a pre-specified range division such as that denoted between the times t0 and t1 in the time waveform 2C. As the current delivered to diodes 72 and 74 through line 78 decreases, the RF attenuation is decreased as a function of the series resistance of the diodes. In practice, this attenuation characteristic is very temperature sensitive requiring complex temperature compensation in the drive circuitry to maintain useful accuracy with fast switching. If a current source is used to improve temperature stability, switching speed is compromised.
Presently, two embodiments have been provided for implementing the PIN diode 50 with microstrip circuit techniques. One embodiment is illustrated in cross-sectional view in FIG. 4 and provides for a chip diode 50 to be hermetically sealed in a conventional diode package 90 which may be disposed in a fitted portion 92 of a metallic microwave substrate, such as aluminum, for example. Normally, the microstrip transmission line 58 may include a soft substrate dielectric 57 such as Epsilam 10 or Duroid for low cost construction and the ground 60 or return may be the aluminum substrate. Microwave chip capacitors 55 and 56 may be positioned on the transmission line surface 58 on either side of the package 90. The chip package 90 may be coupled to the chip capacitors 55 and 56 with a pair of fine wires 96. A primary problem with the embodiment of FIG. 4 is that the chip diode 50 within the package 90 is electrically coupled to the external surface of the package 90 with a thin piece of wire ribbon, for example, as shown at 98. Unfortunately, this thin ribbon 98 provides for appreciable impedance levels at the microwave transmission frequencies being attenuated. As a result, instead of the chip diode 50 providing a short across the lines 58 to 60, an undesirable impedance exists which allows unwanted levels of leakage power to continue on downstream.
Since the cause of the aforementioned problem is generally unavoidable being built in at the manufacturing stage of the package diode, another embodiment excluding the use of packaged diodes appears more attractive. A cross-sectional view of an alternate embodiment is shown in FIG. 5. In this second embodiment, an unpackaged chip diode 50 is disposed in a suitably extruded portion 100 of the aluminum substrate 60 having its cathode 102 electrically coupled to the substrate 60. The anode 104 of the chip diode 50 is typically electrically connected to the capacitor chips 55 and 56 with very fine wire ribbons 106. While maintaining the packaged lead inductance problem of the first embodiment (FIG. 4), this second embodiment (FIG. 5) creates some additional new problems. For example, the connection of the wire ribbon 106 to the chip capacitors 55 and 56, at times, require special processing and care. The length of the bond wires 106 is also critical since this length is designed to match the capacitance of the chip diode 50 in series resonance. As a result of this critical matching, when assembling and integrating the receiver protector with the LNA 18, the exposed bonding wires and semiconductor junctions require special handling and processing.
Another problem surfaced as a result of a high power signal being inadvertently supplied to the input of the receiver protector 22, like when the antenna 16 is accidently disconnected during transmission, for example. Under this unusual condition, the thin wire ribbons 96 or 106, acting as a section of the transmission line 58, has a tendency to burn out because of the high currents passing therethrough, much like a microwave fuse, for example. Consequently, the receiver becomes inoperative to all signals. While protection is afforded, it is one of a destructable protection or one shot protection, resulting in a shut-down of the radar for all practical purposes.
From the above, it is evident that another embodiment is needed for the receiver protector circuit, one which can avoid or alleviate the aforementioned problems and provide even greater radar operational integrity.